The invention relates to method and apparatus for purifying water, particularly in a current efficient manner.
Purified deionized (DI) water is used in a number of analytical applications such as in chromatography and in ion chromatography for making online or offline eluents, in sample preparation applications using auto samplers and in a variety of day to day laboratory uses. Many electrodialytic methods have been used to produce purify DI water.
Electrodialysis as described in the literature is a term used when a cation exchange membrane and an anion exchange membrane are used in conjunction with an electric field for purification of tap water, salt water or brackish water. xe2x80x9cElectrodeionizationxe2x80x9d is the term used to describe the use of ion exchange materials in the above approach. A number of patents discuss the above approaches with various ion exchange materials and configurations.
An early apparatus and method for treating liquids by electrodialysis was shown by Kollsman in U.S. Pat. No. 2,689,826. This device contained anion and cation exchange diaphragms that are arranged in series with electrodes flanking the end of the device. One volume of liquid to be treated is placed in a depletion (purifying) chamber and the ions migrate over time from this volume into a second volume of liquid in the concentration chamber. Thus, the treated liquid is depleted of ions and the liquid in the concentration chamber is enriched with the transferred ions. U.S. Pat. No. 2,794,777 disclosed devices of similar construction with acid and base solutions flowing in the chamber adjacent to the anion and cation diaphragm, respectively. The function of the acid and base solution was to provide a conductive pathway and, thus, lower resistance. The anion and cation exchange diaphragms were walls made with cloth and packed with the respective resin materials. Partial ionization was accomplished with this device in combination with other packed bed columns.
U.S. Pat. No. 2,815,320 disclosed devices that use macroporous ion exchange beads as a filler between permselective anion and cation exchange membranes to lower the resistance and maintain a conductive pathway. There is a suggestion that electrolytes could be circulated in the electrode chambers. According to this patent, the electrolyte in the electrode chambers, and the conductive filler in the intermediate chambers provided a conductive path for current transport. U.S. Pat. No. 2,923,674 disclosed similar electrodialysis devices with multiple anion and cation exchange treatment chambers that facilitate removal of ions and hence purification of a water stream. The above devices also used acid electrolytes in certain chambers.
U.S. Pat. No. 3,149,061 disclosed devices that were useful for removal of both strongly and weakly ionized species from aqueous solutions. The dilution (purifying) chambers in the above devices were either filled with a mixture of cation and anion exchange resins or anion exchange resin by itself. U.S. Pat. No. 3,341,441 disclosed an electrodialytic process where the applied polarity was reversed periodically to minimize scale buildup in the electrode chambers.
U.S. Pat. No. 3,869,376 disclosed devices that were useful for demineralizing soft water. The treatment chamber in these devices were packed with ion exchange resins.
U.S. Pat. No. 4,148,708 disclosed electrodialytic cells that were packed with mixed anion and cation exchange resin in the feed compartment while packing the anode and the cathode compartments with anion exchange and cation exchange resin, respectively. This cell was useful for generating acid, a base and purifier water from the three chambers.
U.S. Pat. No. 4,632,745 disclosed an electrodeionization apparatus with depleting chambers packed with mixed anion and cation exchange resins while the concentration chamber was free of ion exchange resins. Similarly, U.S. Pat. No. 4,925,541 disclosed electrodeionization apparatus with the depletion chambers filled with anion and cation exchange resin beads while the concentration compartments are free of ion exchange beads. The beads in the depleting compartments were housed within subcompartments of controlled width and thickness and were retained therein by ion permeable membranes, which were secured to the wall of the subcompartments. Another version of the above device was shown in U.S. Pat. No. 4,931,160 in which the liquid to be purified was passed through at least two ion depletion compartments filled with anion and cation exchange resin beads. U.S. Pat. No. 4,956,071 disclosed electrodeionization devices that have both the depletion chambers and the concentration chambers filled with ion exchange resin beads. This patent discloses means for reversing the applied polarity and means of recovering a purified product continuously. U.S. Pat. No. 5,154,809 disclosed electrodeionization devices with depletion chambers and possibly concentration chambers filled with mixed ion exchange beads of uniform size.
U.S. Pat. No. 5,308,466 disclosed devices with at least one section of the device having ion exchange membranes/resins of lower crosslinking and lower selectivity, thus reducing the electrical resistance and facilitating removal of large, heavily hydrated, highly or weakly charged molecules (such as silica) from the feed water. U.S. Pat. No. 5,308,467 disclosed electrodeionizers with a radiation grated polymer with mixed ion exchange moieties (anion and cation exchange) as a packing in the demineralizing or dilution compartment.
U.S. Pat. No. 5,736,023 disclosed an electrodeionization apparatus having ion exchange resins in both the depletion and concentration chambers. The apparatus has a polarity reversal means and a means of substituting the fluid in the ion-concentrating compartment with a fluid of lower ionic concentration while maintaining flow in the ion-depleting compartment. U.S. Pat. No. 5,868,915 disclosed electrodeionization apparatus with electroactive media in the depletion and concentration compartments. There are several other patents that disclose improvements to the electrodeionization apparatus and process, such as U.S. Pat. Nos. 6,117,927, 6,126,805, 6,254,753, 6,241,866, 6,241,867 and 6,312,577.
All of the above-disclosed devices are current inefficient devices that require an excess current than predicted from theory for the deionization process. Devices with mixed ion exchange packing materials in the depletion chamber split water and hence are current inefficient.
U.S. Pat. Nos. 6,077,434 and 6,328,885 disclosed means of improving current efficiency for suppressor and suppressor like devices in ion chromatography. The devices disclosed for anion analysis remove cations and convert the matrix ions to a nonconductive form while converting all anion species to conductive acids.
In ion chromatography, the presence of ionic impurities in the water can affect peak response and sensitivity, linearity of response, background stability and baseline noise. The life time of consumables, such as columns and suppressors may also suffer due to the presence of contaminants.
Some of the issues with contaminants in the eluents or reagents may be addressed by using ultra high purity reagents with certified level of contaminants. Analytical laboratories have point of use polisher systems that are intended to lower the level of ionic contaminants in the DI water. With these systems, however, replacing the polisher is not mandated. The detection of water quality on most systems is also not reliable. Use of certified bottled water for eluent or reagent preparation is another approach. This approach, however, suffers from the limitation of contamination from the environment, handling issues, shelf life and added costs. Additionally, it is difficult to eliminate certain contaminants such as carbon dioxide and ammonia, during the reagent preparation process.
The net effect of the above-discussed factors is variability in the water quality from one laboratory to another laboratory. Therefore, it is desirable to have a way to purify DI water online, e.g., for chromatographic systems.
In prior art literature related to electrodialysis and electrodeionization, the current required for purification is derived empirically and typically the applied current is in excess of what is required for purification. This excess current is detrimental as it increases the heat generated by the device or process and the device lifetime suffers due to the excess heat. Therefore, there is a need for a current efficient deionizer device that could self-regulate the current required for the function of purification.
Commercial laboratory water purifiers, such as the devices discussed above, normally operate at very high flow rates in comparison to chromatographic systems. Typically commercial laboratory water purifiers are operated at LPM flow rates, whereas chromatographic systems are operated at ml/min flow rates. Additionally the variance in flow rate is much higher for laboratory water purifiers in comparison to chromatographic systems. This mismatch in flow characteristic results in difficulty in interfacing the two systems.
In normal chromatographic operation water is mixed with eluent and/or solvent using a proportioning valve. Typically, chromatography users fill and replenish the water offline using commercial laboratory water purifier systems. This setup requires frequent monitoring and replenishing of the water purifier and can be quite cumbersome. Due to the high flow rates and pressures of the water stream from commercial laboratory purifiers, it is difficult to directly interface the water stream of the purifier systems to a reservoir of a chromatographic system. One way to resolve this would be to use a sensor that would trigger the purifier to turn off the flow at a given time or in response to some property such as level of liquid in the chromatographic system reservoir. The above approach however is not very practical since most commercial water purifiers begin with a high level of contaminants on starting the purifier and diverting this contaminated water to the chromatographic system reservoir is not acceptable.
In ion chromatographic analysis with a eluent generator module, for example, with module EG40 sold by Dionex Corporation, a pump is fed on the low pressure side with DI water. Eluent is then generated using this module on the high pressure side of the pump. In this mode it would be desirable to feed the pump directly with a stream of DI water. With commercial water purifiers, a direct interface is not possible due to the higher flow rate issue and the issue with contaminants as discussed above. Furthermore, it is very expensive to leave the purifier on all the time. Therefore there is a need for a simple means of connecting commercial water purifiers to an analytical system, such as a chromatographic system or an auto sampler. In particular there is a need for directly interfacing electrodeionizers to chromatographic systems.
In accordance with the present invention, methods and apparatus are provided for removing charged contaminants from a water stream, particularly in a current efficient manner.
Also in accordance with the present invention, methods and apparatus are provided for interfacing a water purifier to an analytical system in a convenient manner.
In one embodiment, the apparatus for removing charged contaminants from a water stream comprises: (a) a first cation exchange membrane with exchangeable cations and a first anion exchange membrane with exchangeable anions both having inner and outer walls, the inner walls of the first cation and anion membranes defining a first purifyng flow channel therebetween, the first cation and anion exchange membranes preventing bulk liquid flow but passing ions of the same charge as the corresponding exchangeable ions (b) a first cation chamber defining a first cation flow channel on the outer wall side of the first cation exchange membrane, (c) a first anion chamber defining a first anion flow channel on the outer wall side of the first anion exchange membrane, (d) a cathode and an anode in electrical communication with the first cation flow channel and the first anion flow channel, respectively, (e) flow-through first ion exchange medium disposed in the first. cation flow channel, (f) flow-through second ion exchange medium disposed in the first anion flow channel and (g) a zone within and coextensive with the length of the first purifying flow channel free of flow-through ion exchange medium or having flow-through ion exchange medium with an ion exchange capacity no greater than 25% of the first and second ion exchange media.
In another embodiment, a method is provided, which may use the foregoing apparatus, in which a water stream is purified by removing contaminants therefrom comprising the steps of: (a) flowing the water stream through a purifying flow channel defined by a cation exchange membrane with exchangeable cations disposed along one side of the purifying flow channel and an anion exchange membrane with exchangeable anions disposed along the other side of the purifying flow channel, the anion and cation exchange membranes preventing bulk liquid flow but passing ions of the same charge as the corresponding exchangeable ions, a zone within and coextensive with the length of the purifying flow channel free of flow-through ion exchange medium or having flow-through ion exchange medium with an ion exchange capacity no greater than 25% of the first and second ion exchange media, (b) flowing an aqueous stream through first ion exchange medium in a cation flow channel on the opposite side of the cation exchange membrane from the purifying flow channel, (c) flowing an aqueous stream through second ion exchange medium in an anion flow channel on the opposite side of the anion exchange membrane from the purifying flow channel, and (d) during steps (a), (b) and (c), applying an electrical potential between a cathode in electrical communication with the cation flow channel and an anode in electrical communication with the anion flow channel.
In a further embodiment, an apparatus for purifying an aqueous liquid stream for flowing the same to an analytical system comprises: (a) a flow-through water purifier having an inlet and an outlet, (b) a pressurized source of impure water communicating with the purifier inlet, (c) an analytical pump, (d) a first conduit disposed between the purifier outlet and the pump, (e) a liquid stream splitter having a diversion outlet, disposed between the impure water source and the purifier inlet, or between the purifier outlet and the analytical pump, the splitter splitting the liquid stream flowing to the splitter inlet into a first and second stream, and (f) a second conduit providing fluid communication between the diversion outlet and the purifier.
In yet another embodiment, a method is provided, which may use the foregoing apparatus, in which a water stream is purified and the same is provided to an analytical system. The method comprises the steps of: (a) purifying the impure aqueous stream by flowing the same from a pressurized source through a purifier having an inlet and an outlet, (b) flowing the purified aqueous stream from the purifier outlet through an analytical pump, (c) splitting the impure aqueous stream between the pressurized source and the purifier so that only part of the impure aqueous stream is purified in the purifier, or splitting the purified aqueous stream from the purifier outlet so that only part of the purified liquid stream flows through the analytical pump.